This invention pertains generally to radar receivers used in semiactive guidance systems, and particularly to an improved narrow band rear receiver for use in such systems.
As is known in the art, a rear radar receiver, the primary function of which is to provide a coherent reference signal (usually at an X-band frequency) for Doppler processing of target return signals in a front radar receiver, is usually incorporated in a semiactive guidance system in a guided missile. To this end, the rear radar receiver is adapted to search for, acquire and track the signal transmitted from an illuminator, thereby to allow generation of a reference signal (offset from the X-band illuminator signal by the first intermediate frequency) for use as the first local oscillator signal of the front radar receiver. In order to maintain the spectral purity of the reference signal in the presence of plume noise, multi-path effects and low rear receiver signal-to-noise ratio so that the subclutter visibility of the front radar receiver may be preserved a narrow band automatic phase control (APC) tracking loop is provided so that the frequency of the rear radar receiver may be locked to the frequency of the illuminator signal,
It has been found to be advantageous to use a so-called YIG filter in the tracking loop. As is known, such a filter is electronically tunable so that compensation may be quickly made for any change in frequency of the illuminator signal as received by the rear receiver. It has been found, however, that in operation, any known YIG filter is very susceptible to vibration and changes in the strength of the required magnetic field. Further, all known YIG filters and the associated drive circuits are relatively expensive.